DESCRIPTION (provided by investigator): The current prospective, longitudinal study will elucidate early visual processing differences in infants with FXS and how abnormal visual processing, if present, relates to cognitive deficits known to emerge in early childhood. Our study will include two comparison groups: infants with Down Syndrome (DS), who will be matched to the FXS group on mental age (MA) and chronological age (CA);and typically developing (TD) infants who will be matched to the FXS group on MA. These comparison groups are crucial for accurately describing the processing abilities in FXS, and differentiating impairments that result from the specific genetic mechanism of FXS, versus generalized effects of mental retardation on brain development. All groups will be seen at two time points. To examine the developmental changes in visual and cognitive processing, the Time 2 visit will include the repetition of some of the experimental paradigms and standardized assessments done at time 1, as well as other assessments of cognitive functioning. An important aspect to understanding the early phenotypic features of FXS is determining how these features do and do not change with the co-morbid diagnosis of autism. For this reason, the Time 2 assessment will also provide a comprehensive standardized assessment of autism as well as documentation of the emergence of other behavioral and cognitive problems. Our first specific Aim is to determine whether deficits exist in low level visual processing in infants with FXS. Our second Aim explores whether deficits exist in higher level visual processing in infants with FXS;particularly, to explore the hypothesis that problems exist with parietally-mediated, but not temporally-mediated visual functions. Aim 3 is to determine whether patterns of visual processing observed in infants predict later visual and cognitive processing. Finally, Aim 4 is in place to determine whether behavior on low level or high level visual processing, or later cognitive processing, is predicted by the co-morbid diagnosis of Autism Spectrum Disorders (ASD) in late toddlerhood. Ours will be the first study to address the development of visual (and related cognitive) processing in infants with FXS. The information gained from this research can be utilized in future studies of early interventions using behavioral, medical and education-based treatments. The need for these studies is particularly pressing, given the potential for newborn screening procedures to be adopted for FXS.